Golang internal/trace/traceview: panic if try to handle plot empty mmu slice

Go version

go version 1.24.4 on linux

Output of `go env` in your module/workspace:

AR='ar'
CC='gcc'
CGO_CFLAGS='-O2 -g'
CGO_CPPFLAGS=''
CGO_CXXFLAGS='-O2 -g'
CGO_ENABLED='1'
CGO_FFLAGS='-O2 -g'
CGO_LDFLAGS='-O2 -g'
CXX='g++'
GCCGO='gccgo'
GO111MODULE=''
GOAMD64='v1'
GOARCH='amd64'
GOAUTH='netrc'
GOBIN=''
GOCACHE='/home/tiago/.cache/go-build'
GOCACHEPROG=''
GODEBUG=''
GOENV='/home/tiago/.config/go/env'
GOEXE=''
GOEXPERIMENT=''
GOFIPS140='off'
GOFLAGS=''
GOGCCFLAGS='-fPIC -m64 -pthread -Wl,--no-gc-sections -fmessage-length=0 -ffile-prefix-map=/tmp/go-build3237773918=/tmp/go-build -gno-record-gcc-switches'
GOHOSTARCH='amd64'
GOHOSTOS='linux'
GOINSECURE=''
GOMOD='/dev/null'
GOMODCACHE='/home/tiago/work/go/pkg/mod'
GONOPROXY='gitlab.nereides.weborama.com'
GONOSUMDB='gitlab.nereides.weborama.com'
GOOS='linux'
GOPATH='/home/tiago/work/go'
GOPRIVATE='gitlab.nereides.weborama.com'
GOPROXY='https://proxy.golang.org,direct'
GOROOT='/usr/local/go1.24.4'
GOSUMDB='sum.golang.org'
GOTELEMETRY='on'
GOTELEMETRYDIR='/home/tiago/.config/go/telemetry'
GOTMPDIR=''
GOTOOLCHAIN='auto'
GOTOOLDIR='/usr/local/go1.24.4/pkg/tool/linux_amd64'
GOVCS=''
GOVERSION='go1.24.4'
GOWORK=''
PKG_CONFIG='pkg-config'

What did you do?

while I was trying to analyse a 5s trace from my application, using the snipped presented on https://pkg.go.dev/net/http/pprof

curl -o trace.out http://localhost:6060/debug/pprof/trace?seconds=5
go tool trace trace.out

What did you see happen?

the tool open my default browser in the trace page

When I click on the link Minimum mutator utilization that uses endpoint /mmu I got a modal with an error (see screenshot below) and I can see the following panic stack trace:

2025/06/13 14:43:05 http: panic serving 127.0.0.1:44114: runtime error: index out of range [0] with length 0
goroutine 77 [running]:
net/http.(*conn).serve.func1()
    net/http/server.go:1947 +0xbe
panic({0x9324c0?, 0xc000028330?})
    runtime/panic.go:792 +0x132
internal/trace/traceviewer.(*mmu).HandlePlot(0xc000236030, {0xcbf2f8, 0xc002a40460}, 0xc002a3e140)
    internal/trace/traceviewer/mmu.go:140 +0x9a6
main.main.MMUHandlerFunc.func21({0xcbf2f8, 0xc002a40460}, 0xc002a3e140)
    internal/trace/traceviewer/mmu.go:52 +0x67
net/http.HandlerFunc.ServeHTTP(0xc0002320c0?, {0xcbf2f8?, 0xc002a40460?}, 0x761436?)
    net/http/server.go:2294 +0x29
net/http.(*ServeMux).ServeHTTP(0x474d79?, {0xcbf2f8, 0xc002a40460}, 0xc002a3e140)
    net/http/server.go:2822 +0x1c4
net/http.serverHandler.ServeHTTP({0xc0000c9c80?}, {0xcbf2f8?, 0xc002a40460?}, 0x6?)
    net/http/server.go:3301 +0x8e
net/http.(*conn).serve(0xc002a60090, {0xcbfa30, 0xc0002361b0})
    net/http/server.go:2102 +0x625
created by net/http.(*Server).Serve in goroutine 1
    net/http/server.go:3454 +0x485

To be honnest, this is not a big issue, but it is annoying

By check go source code, I find the possible root cause

https://cs.opensource.google/go/go/+/refs/tags/go1.24.4:src/internal/trace/traceviewer/mmu.go;l=140

I believe the mu slice is empty, then when try to fetch mu[0] it panics.

probably some basic length check can be done to prevent it. I could try to send a Pull Request however I have no idea what should be the output (perhaps a 404 ?)

What did you expect to see?

No panic and perhaps a nice message about "having no data to show"

Comment From: peczenyj

this is the trace file, in case it is needed: trace.out.zip

Comment From: gabyhelp

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Comment From: cherrymui

cc @golang/runtime @mknyszek @cagedmantis

Comment From: mknyszek

Huh. I'm slightly worried there's a deeper bug here, that a simple length check would just end up hiding. I'll take a look at the trace file and try to see what the appropriate behavior here should be based on the trace data.